Methods for the electrodeposition of metals



United States Patent 3,039,943 METHODS FOR THE ELECTRODEPOSITION OF METALS George Chandler Cox and Walter Elwood Vail, Charleston, W. Va., assignors to George Chandler Cox No Drawing. Filed Oct. 1, 1959, Ser. No. 842,021 6 Claims. (Cl. 20438) This invention relates to improved methods and processes for electrolytically depositing metals without the necessity of immersing the object to be electroplated in an aqueous solution containing a soluble compound of the metal to be deposited, whereby the cost of electrolyte is greatly reduced and the quality of the deposit and the throwing power are improved.

Other than as disclosed in the copending Cox application Serial No. 690,640, now US. Patent No. 2,919,233, as far as is known no method has been developed for electroplating a conducting object or structure except when the object is immersed in or is in contact with a solution of a compound of the metal which it is desired to deposit. In the above-mentioned copending application, methods have been described whereby the amphoteric metals lead, tin and zinc may be electrolytically deposited on a cathodic metal structure coated with a composition containing a relatively insoluble compound of the metal to be deposited when the structure to be plated is immersed in an electrolyte containing a salt of an alkali metal but containing no soluble compound of the metal to be deposited.

The object of the present invention is to accomplish the electrolytic deposition of certain metals, either singly or as alloys, from coatings of their relatively insoluble compounds on a cathodic structure immersed in or in contact with an electrolyte containing no soluble compound of the metal or metals to be deposited but containing one or more soluble compounds that will incrementally solubilize the metal components of the coating by formation of compleX ions. Complexing compounds which may be used in electrodepositing metals by this process include ammonia, the ammonium salts, the pyrophosphates, the cyahides, the fiuoborates, the fluosilicates and the thiosulfates, and various chelating agents. Metals which we have electrodeposited from coatings containing their relatively insoluble compounds when the electrolyte has contained one or more of the aforementioned complexing compounds include: copper, silver, zinc, cadmium, tin, lead, cobalt, and nickel. These metals have been deposited through the employment of reactions other than the amphoteric reactions given in copending application No. 690,640, now US. Patent 2,919,233.

Another object is to electrolytically deposit copper, silver, zinc, cadmium, cobalt or nickel, or alloy mixtures from this group, from a composition which has been applied as an adherent coating to a conducting structure and which contains compounds of low solubility of the specific metal or metals to be deposited, and from which the aforesaid specific metal content is incrementally solubilized and cathodically deposited on said conducting structure during passage of current through an electrolyte that contains ammonia or an ammonium salt.

Another object is to electrolytically deposit copper, silver, zinc, cadmium, tin, lead, cobalt or nickel from a composition which has been applied as an adherent coating to a conducting structure and which contains compounds of low solubility of the specific metal or metals to be deposited, and from which the aforesaid specific metal content is incrementally solubilized and cathodically deposited on said conducting structure during passage of current through an electrolyte that contains a pyrophosphate of an alkali metal.

Another object is to electrolytically deposit copper, sil- 3,039,943 Patented June 19, 1962 "ice ver, zinc or cadmium, or alloy mixtures from this group, from a composition which has been applied as an adherent coating to a conducting structure and which contains compounds of low solubility of the specific metal or metals to be deposited, and from which the aforesaid specific metal content is incrementally solubilized and cathodically deposited on said conducting structure during passage of current through an electrolyte that contains a cyanide of an alkali metal.

Another object is to electrolytically deposit certain metals either singly or in combination on a conducting structure from an adherent coating composition which has] been applied to said structure and which contains compounds of low solubility of the metal or metals to be deposited, and incrementally to solubilize the metal component of this adherent coating during passage of a current through an electrolyte containing a fluoborate, fluosilicate or a thiosulfate of an alkali metal, or mixtures thereof, while at the same time electrodepositing on said structure the desired metal or metals from the solubilized. product.

Another object is to electrolytically deposit one or more metals from the group consisting of copper, silver, zinc, cadmium, tin, lead, cobalt and nickel from a composition which has been applied as an adherent coating to a conducting structure and which contains compounds of low solubility of the specific metal or metals to be deposited, and from which the aforesaid specific metal content is incrementally sclubilized and cathodically deposited on said conducting structure during passage of current through an electrolyte that contains two or more com-. plexing agents as hereafter listed in Section II.

From the foregoing objects it is apparent that these procedures consist of two distinct and essential parts: (I) the methods of forming on an electrically conducting structure a suitable adherent coating of low solubility containing a compound of the metal to be electrolytically deposited when the structure is subsequently made cathodic, and (II) the methods of incrementally solubilizing such a coating at the cathode interface so that the desired metal can be electrolytically deposited.

For purposes of simplifying the language, the following definitions will be used throughout this application:

An adherent coating will be defined as an adherent, non-metallic coating which can be formed by any of the various methods herein discussed on an electrically conducting structure, which is a coating containing one or more compounds of low solubility of the metal to be deposited and which is permeable to the electrolyte.

It is intended that the ,conducting structure to be electroplated be defined as including any thoroughly clean electrically conducting structure, either metallic or non-metallic. Also, it is intended that ferrous metal structure be defined as including the various steels and any other alloy of iron which has or has not been electroplated with one or more other metals, or otherwise surface coated with a metal as by dipping, cladding, and the like.

1. Methods of Forming an Adherent Coating An adherent coating may be formed on a conducting structure in a number of ways, four of which are illustrated in the following examples, the choice of method depending on the nature of the conducting structure and the metal to be deposited:

(a) As a first step in preparing such an adherent coating on an electrically conducting structure, the structure may be coated with a basic coating of low'solubility which does not contain the metal to be deposited in accordance with the teachings of US. Patents No. 2,200,469 and No. 2,534,234. For this step the forming electrolyte would be either sea water or a solution containing magnesium ions. Because of the desirability of a porous deposit for the present invention the need for calcium ions for densification as discussed in these patents is not a requirement. Also, in operation under the teaching of these patents use of the high magnesium field for increased porosity of the basic deposit is desirable. As described in later paragraphs, the initial basic coating of low solubility may then be treated by dipping, spraying or otherwise wetting the coating under controlled conditions with a soluble salt of the metal to be deposited, such as the chloride, nitrate or sulphate. The controlled conditions would include such items as concentration of metal ion, concentration of hydrogen ion, time and temperature. As a result of these steps an adherent coating containing a compound of low solubility of the metal can be formed.

If it is desired to deposit lead when using the procedure described in paragraph (a) above, the soluble lead salt may be either lead nitrate or lead acetate. For example, when a panel coated with an initial basic coating of low solubility was treated for one hour at room temperature with a normal solution of lead nitrate having a pH value of 3.8 the weight gain was 0.35 gram. No appreciable further gain in weight was obtained when the treating time was extended up to 48 hours, and for treating times materially less than one hour the weight gain dropped off rather rapidly. Higher temperature treatments resulted in a material reduction in the treating time for the maximum weight gain. This eifect of time and temperature in general was found for other metals also. For example, a maximum weight gain was obtained on a coated panel when it was threaded for five minutes at 75 :2 C. with a normal solution of zinc chloride having a pH of 5.5. A maximum weight gain with zinc sulphate was likewise obtained under these conditions. When panels coated with an initial basic coating of low solubility were treated at 83 :2" C. for two hours in a normal solution of nickelous chloride having a pH of 6.4 useful weight gains were obtained.

When normal stannous chloride solutions were used at room temperatures for treatment of the initial basic coating described in paragraph 1(a), the low pH of approximately 1.0 resulting from hydrolysis of the chloride caused solution of the initial basic coating described in about seconds. However, when tenth normal stannous chloride solutions having pH values of approximately 2.1 were used at room temperature fairly good adherent coatings containing tin were obtained with treatments of about two minutes. Illustrating the effect of higher pH values, when similarly coated panels were treated at room temperature (19 C.) for six hours in a normal solution of cadmium chloride having a pH value of 5.7 useful weight gains were obtained. For any particular metal the optimum combination of temperature time, acidity and nietalion concentration can readily be determined. In general, the pH of the metal salt solution should be adjusted to incipient precipitation.

It should be emphasized that room temperature conditions are highly desirable for low cost treatment of large tanks and structures, but it is explicitly understood that the examples given in this application do not constitute a limitation in any way whatever on any of the various reaction conditions and variables involved in the procedures herein described.

(b) If it is not convenient to give the structure an initial basic coating of low solubility in accordance with the teachings of the above-mentioned patents, a composition for such a coatingmay'be conveniently made by mixing in proper proportions magnesium hydroxide and magnesium chloride to give a composition related to the well known magn'esium-oxychloride cements. Then a suitably cleaned conducting structure may be coated with a slurry of this compositionby any of the standard procedures such as by spraying, dipping or painting and allowing it to harden into an adherent coating which'does not as yet contain the metal to be electrodeposited. Other chemical compositions for the initial coating may similarly be use, provided they are somewhat basic, such as a calcium cement. Then a, desired metal content may be chemically precipitated by wetting the coated surface of the structure with a solution containing a metal salt under controlled conditions as discussed above, thereby forming the desired adherent coating.

(c) A hydroxide or other compound of low solubility of a metal to be deposited may be made into a cement by the use of suitable inorganic or organic bonding agents. When it is desired to electrodeposit zinc on a conducting structure, the structure may be coated with a slurry of a freshly made zinc oxychloride or phosphate cement. For example, a slurry of 40% ZnO, 12% ZnCl and 48% water by weight was pasted on copper panels and allowed to harden overnight and electrolyzed as described in Section 11 into a good deposit of zinc. When lead is the desired metal to be deposited a suitably thin slurry of litharge and glycerin may be applied to form an adherent coating by any of the usual methods (spraying, dipping or painting) and then allowed to harden. For example, a slurry of 78% litharge and 22% glycerin by weight was pasted in a thin film onto mild steel panels and allowed to harden overnight before being electrolyzed as discussed in Section II.

(d) An adherent coating of low solubility containing any of the metals of this invention may be formed on a conducting structure by adding the necessary amount of a very finely divided relatively insoluble compound of the desired metal to a magnesium-oxychloride cement slurry, and then applying this slurry as a thin coating over the surface to be electroplated and allowing it to harden. For example, a slurry of 27% MgO, 45% MgCl .6H O, 10% NiO and 18% water was applied as a slurry, allowed to harden overnight, and electrolyzed as described in Section II to give a deposit of nickel. Other cements, such as Portland cement, or a suitable inorganic or organic bonding agent may be used as a binder, provided that no shrinkage cracks oc our in the cement coating when it has set, and provided that it is permeable to the electrolyte subsequently used in plating.

II. Methods of Solubilizing the Metal Content of an Adherent Coating One of the essential features of this invention is that the metal content of an adherent coating is slowly solubilized at the cathode-coating interface, thereby providing a low concentration of the required metal ions from which the desired metal, or metals, is subsequently deposited electrolytically. For, in accordance with present electroplating theory, a requirement for producing dense, fine-grain deposits of low porosity is that the metal ion concentration must be maintained at all times during electrolysis at a very low value in comparison to the total available metal content which in this invention is to be electroplated from the adherent coating composition.

One procedure which fulfills these conditions has been disclosed and claimed in the copending application US. Serial No. 690,640 as follows: The process of electrolytically depositing an amphoteric metal, selected from the group consisting of lead, tin and Zinc, on a ferrous metal structure which comprises coating said structure with a firmly adherent amphoteric coating complex of low solubility containing the metal to be deposited, and then subjecting said coated structure to a cathodic current density of sufiicient magnitude. in an electrolyte containing a soluble alkali metal salt which "ill liberate a hydroxide of the alkali metal at the cathode thereby 'solubilizing the amphoteric metal component of the amphoteric coating complex at the cathode interface while at the same time electrodepositing on the cathode the selected amphoteric metal from the solubilized prodnot.

What we now propose and claim in this application, in addition to some methods of forming adherent coatings not disclosed in Serial No. 690,640, are procedures for incrementally solubilizing the metal content of an adherent coating by the formation of complex ions.

The following examples will illustrate various methods of solubilizing the metal compound of an adherent coating by the use of complex ions, and electrolytically depositing the metal.

(a) We have obtained excellent deposits of metal in cases where complex cations containing the metal have been formed at the cathode-coating interface. A distinct advantage in the use of complex cations, rather than anions, is that the cation migrates toward the cathode and a more efficient electrode reaction results than when a soluble anion containing the metal is formed in the coating and begins migrating toward the anode. For several metals an electrolyte containing ammonia or an ammonium salt is very satisfactory for progressively solubilizing the metal compound in the coating as a soluble complex cation. Such ions include Ag(NH Cd- 3)4 CO(NH3)G++, a)4 3)6 Zn(NH )4 from which we have found that the metals can be electroplated on a cathode by the methods herein disclosed. These are the formulas usually assigned, though in aqueous solution there are probably also ions containing other numbers of molecules of ammonia than here indicated.

Specifically, when an adherent coating containing nickel Was formed on a copper panel at 83 i2 C. from nickel chloride, as outlined in foregoing paragraph 1(a), and, after washing, was immersed in a mixed bath of N/l sodium chloride and N/ ammonium chloride and then subjected to a cathodic current density of 50 ma./ sq. ft. for 48 hours at 19:? C. an excellent highly polished, bright, dense deposit of nickel was obtained. Also, when an adherent coating containing zinc was formed on a copper panel from a normal zinc sulphate solution for 24 hours at 23i1 C., as outlined in paragraph 1(a), and, after washing, was immersed in a mixed bath N/ l with respect to sodium chloride and N/10 with respect to ammonium chloride and then subjected to a cathodic current density of 50 ma./sq. ft. for 71 hours at 2211 C. an excellent dense deposit of zinc was obtained. When another copper panel with an adherent coating containing zinc was electrolytically reduced under exactly the same conditions as the last-mentioned panel above except that sea water with N/ 10 ammonium chloride was used as the electrolyte, the deposit of zinc was almost as good as the panel reduced in a mixed bath N/l with respect to sodium chloride and N/lO with respect to ammonium chloride. When an adherent coating containing cobalt was formed on a copper panel from 21 normal cobaltous sulphate solution during 11 hours treatment at room temperature, as outlined in paragraph 1(a), and, after washing, immersed in a bath of N/l ammonium chloride and then subjected to a cathodic cur- V rent density of 100 ma./sq. ft. for 24 hours at room temperature an excellent smooth, bright, uniform deposit of cobalt was obtained. The liberation of ammonia at the cathode occurred throughout this entire test.

We have also electroplated copper on a graphite electrode from an adherent coating in an ammonium chloride electrolyte. The adherent coating was formed as outlined in paragraph 1(a), by treating an initial basic coating for 30 minutes at 80:3" C. with a solution of N/ 1 copper sulphate having a pH of 4.0. The coating was then electrolyzed at ma./sq. ft. for 24 hours at room temperature in N/ 10 ammonium chloride solution, whereby a thin but quite uniform deposit of copper was plated out on the graphite. Here, as in all cases with an electrolyte containing ammonium chloride, ammonia was liberated at the cathode throughout the test.

(b) We have incorporated in the adherent coating a hydroxide or other relatively insoluble compound of the metal to be deposited and progressively solubilized it at the interface as a complex anion by the use of an electrolyte containing an alkali metal cyanide, pyrophosphate, or salt of an organic acid such as a citrate, oxalate or tartrate. Examples of such complex anions containing the metal are:

The following examples illustrate this type of reaction: When an adherent coating containing lead was formed at 75 -1 C. as described in foregoing paragraph 1(a) and then immersed at room temperature in an M/ 8 solution of sodium pyrophosphate and made cathodic at 50 ma./ sq. ft. for 48 hours a bright, silvery, fine-grain, well bonded and very uniform deposit of lead was obtained. When an adherent coating containing zinc was formed as given in paragraph 1(a) above, then immersed in an M/8 solution of sodium pyrophosphate and made cathodic at ma./sq. ft. for 48 hours a bright, silvery, fine-grain, well bonded and very uniform deposit of zinc was obtained. An excellent electroplate of cadmium was obtained when a panel covered with a cadmium-containing adherent coating was similarly electrolyzed in sodium pyrophosphate. Good uniform deposits of lead were obtained when panels were covered with a lead-containing adherent coating made in accordance with the teaching of foregoing paragraph 1(a) and then were cathodically reduced at 25, 50, 100 and 200 ma./ sq. ft. for 24 hours in an M/ 5 solution of sodium pyrophosphate. Good deposits of tin also were obtained in pyrophosphate electrolytes.

Furthermore, when copper panels covered with tincontaining adherent coatings formed as in paragraph 1(a) were cathodically reduced at 100 ma./ sq. ft. for 24 hours in normal solutions of sodium cyanide good deposits were obtained. Also, in a cyanide bath,.we have electroplated silver on a graphite electrode from an adherent coating. The adherent coating was formed as outlined in paragraph I(a) by treating an initial basic coating for one hour at room temperature in a N/ 1 silver nitrate solution of which the pH had not been adjusted. The coating was then electrolyzed at 75 ma./sq. ft. for 24 hours at room temperature in a solution which was N/ 10 with respect to both sodium cyanide and sodium hydroxide, whereby a thin fairly uniform deposit of silver was plated out on the graphite cathode.

We have electroplated lead on a mild steel cathode from an adherent coating in an electrolyte containing a tartrate. The adherent coating was formed as outlined in paragraph 1(a) by treating an initial basic coating for ten minutes at 77i1 C. in a N/ 1 solution of lead nitrate having a pH of 4.6. The coating was then electrolyzed at 25 ma./ sq. ft. for 24 hours at room temperature in M/S sodium potassium tartrate, whereby a thin, bright uniform plating of lead was formed on the steel panel.

General These procedures allow a conducting structure to be electroplated with any metal which can be placed on the structure as an adherent coating of controlled thickness and then incrementally solubilized through the formation of complex ions and cathodically deposited from the solubilized product. As previously indicated, we believe one explanation of the fine grain and low porosity of deposits which we have obtained by the use of complex ions for solubilizing the metal content of an adherent coating is that they provide an extremely small concentration of metal ions for electrolytic reduction at the cathode as has been found desirable in use of these complex ions in ordinary electroplating baths. However, the increase in throwing power and covering power which can be obtained in the electrodeposition of metals by our procedures is probably due largely to the uniform distribution of the adherent coating over the entire surface being electroplated. This results in a more equal distribution of the metal to be deposited than is possible where metal ion depletion or a decrease in current density at a remote or shielded point will cause serious variations in metal deposition. Another probable reason for the improved results which are characteristic of these processes is the relatively high resistance and consequent voltage drop across the adherent coating, which in effect amounts to a greater cathode polarization than that ordinarily obtained in the customary plating bath. This increase in effective polarization would be expected to cause a decrease in crystal size of the metal deposit, a decrease in treeing, and an increase in throwing power.

Within reasonably Wide limits of cathodic current density, from 10 ma./sq. ft' to 12.5 amp/sq. ft. and with tenth normal up to normal solutions of one or more compounds that will cause complex ion formation under the conditions herein described, we have obtained firmly bonded electrolytic deposits of the desired metal on a cathode, although the best deposits formed by any of the methods herein disclosed were made at low current density with low electrolyte concentrations. Higher temperatures of the reduction baths generally allow the use of higher current densities.

We have obtained excellent deposits at current densities between 50 and 200 ma./sq. ft. and have observed no falling off in quality of deposit at current densities as low as 10 ma./sq. ft. The maximum current density should not be so high that the metal deposit will be burned, spongy, non-uniform or arboreal in structure. The optimum current density wih depend on such factors as temperature, resistance across the adherent coating and on the nature of the cathode, of the metal being deposited, and of the electrolyte.

The concentration of the complexing compound in the electrolyte must not be so great as to dissolve the adherent coating rapidly and allow it to difius away from the cathode before there has been time for the metal ion to be deposited electrolytically thereon. This precaution is more necessary with those electrolytes which produce complex anions containing the metal than with those which form complex cations. When the electrolyte contains an ammonium salt, the major solubilizing effect occurs only with the flow of current, which liberates ammonia at the cathode with subsequent formation of soluble metal amrn-ines. For production of plate of high quality the optimum concentration of complexing compound in the electrolyte will depend on the nature of the metal being deposited, on the nature of the cathode surface, and on such variables as current density, temperature, pH and the amount of non-complexing salt that may also be present in the electrolyte.

While the preferred embodiments of the invention are herein disclosed by way of example, it is understood that the invention is not limited with respect to the precise modes of forming an initial basic coating of low solubility, to the modes of forming an adherent coating of low solubility containing the metal to be deposited, or to the modes of solubilizing and electro-depositing the metal content of the adherent coating, within the spirit of the appended claims.

We claim:

1. The process of electrolytically depositing on a conducting structure a metal selected from the group consisting of copper, silver, zinc, cadmium, cobalt and nickel which comprises applying to said structure an adherent coating containing a compound of low solubility of the metal to be deposited, and then subjecting said coated structure to a cathodic electromotive force in an aqueous electrolyte containing an ammonium salt which will liberate ammonia at the cathode interface thereby incrementally solubilizing the selected metal component of the coating as a soluble metal ammine complex ion while at the same time the selected metal is electrodeposited on the cathode from the solubilized product.

2. The method of claim 1 wherein the conducting structure is a metal structure.

3. The method of claim 1 wherein the conducting structure is a ferrous metal structure.

4. The process of electrolytically depositing on a conducting structure a metal selected from the group consisting of copper, silver, zinc, cadmium, cobalt and nickel which comprises applying to said structure an adherent coating containing a compound of low solubility of the metal to be deposited, and then subjecting said coated structure to a cathodic electromot-ive force in an aqueous electrolyte containing ammonia thereby incrementally solubilizing the selected metal component of the coating as a soluble metal ammine complex ion while at the same tim electrcdepositing on the cathode the selected metal rom the solubilized product.

5. The method of claim 4 wherein the conducting structure is a metal structure.

6. The method of claim 4 wherein the conducting structure is a ferrous metal structure.

Harford Aug. 8, 1944 Cox Dec. 29, 1959 

1. THE PROCESS OF ELECTROLYTICALLY DEPOSITING ON A CONDUCTING STRUCTURE A METAL SELECTED FROM THE GROUP CONSISTING OF COPPER, SILVER, ZINC, CADMIUM, COBALT AND NICKEL WHICH COMPRISES APPLYING TO SAID STRUCTURE AN ADHERENT COATING CONTAINING A COMPOUND OF LOW SOLUBILITY OF THE METAL TO BE DEPOSITED, AND THEN SUBJECTING SAID COATED STRUCTURE TO A CATHODIC ELECTROMOTIVE FORCE IN AN AQUEOUS ELECTROLYTE CONTAINING AN AMMONIUM SALT WHICH WILL LIBERATE AMMONIA AT THE CATHODE INTERFACE THEREBY INCREMENTALLY SOLUBILIZING THE SELECTED METAL COMPENENT OF THE COATING AS A SOLUBLE METAL AMMINE COMPLEX ION WHILE AT THE SAME TIME THE SELECTED METAL IS ELECTRODEPOSITED ON THE CATHODE FROM THE SOLUBILIZED PRODUCT. 